In your Controller, right above your init() method, you can add a 'refs' array:

refs: [{
ref: 'userInsert',
selector: 'userinsert'
}],

In your Controller, you'll then 'auto-magically' have access to method this.getUserInsert(), which returns a reference to your userinsert widget. In your click handler method this.insertUser(), you can then do something like this: